Abstract: Aquifer Mineralogy and Natural Radionuclides in Groundwater - The Lower Paleozoic of Central Texas

Yongje Kim, Thomas T. Tieh, Ernest B. Ledger

Water-mineral interactions in an aquifer may give rise to high levels of Ra and Rn in groundwater. An understanding of aquifer mineralogy is therefore essential to determine the sources of natural radionuclides and design possible means for improving water quality. Anomalous Ra and Rn concentrations have been detected in groundwater produced from the Cambrian Hickory and Cap Mountain aquifers in the Llano Uplift area of central Texas. This study examines cored aquifer rock samples, focusing on the abundance, distribution, and nature of occurrence of minerals containing U and Th, parent nuclides of Ra and Rn.

The Hickory, 136 m thick, consists of a coarse grained lower, a calcareous middle, and a fossiliferous and hematitic upper unit, with thin shale laminae throughout. The Cap Mountain, 44 m thick, is a sandy limestone. Detrital materials are composed of 77 % quartz, 19 % feldspars, and 4 % lithic fragments. Accessory minerals average less than 1 %. Authigenic minerals, primarily carbonate, clay, and Fe-oxide minerals, make up 18 % of the bulk rock. Porosity is of secondary origin.

Analysis of U in 123 samples by delayed neutron counting shows an average concentration of 3.6 ppm. Shaly samples generally contain significantly higher U. Gamma ray spectrometric analysis of Th in 20 samples yields an average of 13.9 ppm. Fission-track imaging shows that U occurs predominantly in: (1) phosphatic fossil fragments and intraclasts, especially in the Cap Mountain; (2) thin shaly laminae which are more abundant in the Hickory; (3) authigenic minerals including hematite and clay minerals, also common in the Hickory; and (4) detrital accessory minerals.

Mobilization of U and its decay products by groundwater can account for the Ra and Rn in the produced water, particularly from intervals where there are high concentrations of shaly laminae, phosphatic materials, or hematitic cement.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana